Laser diodes are widely used in optical communications systems to transmit optical signals because of their desirable characteristics, such as high output power, narrow spectral width and fast switching speed. Unfortunately, laser diodes are intolerant of high temperatures. Currently, the performance of a laser diode typically degrades sharply as the operating temperature rises to around 80 degree Celsius. Thus, the operating temperature of a laser diode should be maintained at a temperature below 80 degrees Celsius to ensure that the laser diode performs at a desirable level.
One conventional technique to maintain the operating temperature of a laser diode below a prescribed temperature, e.g., 80 degrees Celsius, is to thermally connect a cooling device, such as a heat sink or a thermoelectric cooler, to the laser diode. The cooling device operates to dissipate the heat from the laser diode to reduce the operating temperature of the laser diode such that the laser diode can be maintained below the prescribed temperature. However, the operating temperature of a laser diodes is not only dependent on the heat generated by the laser diode itself but also on the heat generated by electrical components in close proximity to the laser diode. In particular, the heat generated by an output transistor of an integrated circuit (IC) device for driving the laser diode contributes a significant amount of heat to the laser diode, increasing the operating temperature of the laser diode. Due to the heat contribution of the output transistor of the IC device to the operating temperature of the laser diode, the cooling device may not be able to maintain the operating temperature of the laser diode below the prescribed temperature. Alternatively, the required size of the cooling device to maintain the operating temperature of the laser diode below the prescribed temperature may exceed a practical limit when the heat contribution from the output transistors is taken into consideration. Therefore, reducing the heat contribution of the output transistor of the IC device to the operating temperature of the laser diode is desirable.
One solution to reduce the heat contribution of the output transistor of the IC device to the operating temperature of the laser diode is to increase the distance between the output transistor and the laser diode. However, due to inductive effect, the output transistor must be placed very close to the laser diode for high speed applications. In addition, there is an increasing demand to reduce the size of the overall product and increase port density. Thus, increasing the distance between the output transistor and the laser diode is not, in general, a practical solution.
Another solution is to decrease the power dissipation of the output transistor to correspondingly decrease the heat generated by the output transistor. However, a laser diode typically require high driving currents, and consequently, the high driving currents must be passed through the output transistor of the IC device to drive the coupled laser diode.
In view of these constraints, what is needed is an IC device for driving a laser diode that reduces the amount of heat transferred from an output transistor of the IC device to the laser diode without reducing the driving currents supplied to the laser diode.